Valves are mechanical devices that controls the flow and pressure within a system or process. They are essential components of a piping system that conveys liquids, gases, vapors, slurries etc..
Different types of valves are available. Gate, globe, plug, ball, butterfly, check, diaphragm, pinch, pressure relief, control valves etc. Each of these types has a number of models, each with different features and functional capabilities. Some valves are self-operated while others manually or with an actuator or pneumatic or hydraulic is operated.
There are many valve designs, types and models, with a wide range of industrial applications. All satisfy one or more of the functions identified above. Valves are expensive items, and it is important that a correct valve is specified for the function, and must be constructed of the correct material for the process liquid.
Regardless of type, all valves have the following basic parts: the body, bonnet, trim (internal elements), actuator, and packing.
The valve body, sometimes called the shell, is the primary boundary of a pressure valve. He serves as the main element of a valve assembly because it is the framework that holds all the parts together.
The body, the first pressure boundary of a valve, resists fluid pressure loads from connecting piping. It receives inlet and outlet piping through threaded, bolted, or welded joints.
The cover for the opening in the body is the bonnet, and it is the second most important boundary of a pressure valve. Like valve bodies, bonnets are in many designs and models available.
The removable and replaceable valve internal parts that come in contact with the flow medium are collectively termed as Valve trim. These parts include valve seat(s), disc, glands, spacers, guides, bushings, and internal springs. The valve body, bonnet, packing, et cetera that also come in contact with the flow medium are not considered valve trim.
Disk and Seat
The disc is the part which allows, throttles, or stops flow, depending on its position. In the case of a plug or a ball valve, the disc is called plug or a ball. The disk is the third most important primary pressure boundary. With the valve closed, full system pressure is applied across the disk, and for this reason, the disk is a pressure related component. Disks are usually forged, and in some designs, hard surfaced to provide good wear properties. Most valves are named, the design of their disks.
The valve stem provides the necessary movement to the disc, plug or the ball for opening or closing the valve, and is responsible for the proper positioning of the disk. It is connected to the valve handwheel, actuator, or the lever at one end and on the other side to the valve disc. In gate or globe valves, linear motion of the disc is needed to open or close the valve, while in plug, ball and Butterfly valves, the disc is rotated to open or close the valve. It can give you an impression, and good understanding of the differences between the various types of valves, and how these differences affect the valve function. It will help to a proper application of each type of valve during the design and the proper use of each type of valve during operation.
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